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Author Topic: Gain control now clean like master volume in Prince Boogie copy amp.  (Read 20446 times)

phatt

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #45 on: June 25, 2012, 04:13:37 AM »
Without a schematic this will go on forever.

If this Amp means that much to you then,,Time to put in the hard yards and back engineer the circuit and draw up a schematic.

If you Think I'm cruel? :loco

Yes it's bloody hard to do but I probably knew less than you when I started and   
Oh Yes,,,did I mention,,, There was NO internet for Me.   :'(
So if I can do it?  :tu:

Phil.

Paolo

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #46 on: June 25, 2012, 08:23:45 AM »
No Phil, i don't think you're being cruel, but you said a similar thing back on page 1. 

Since then we have got the gain working and sounding great; we have discovered that putting in a different reverb tank vastly improves the reverb; we have found Roly!  There is a minor gliche in the circuit that still needs to be ironed out and we are working our way through some ideas.

One day i hope to understand schematics, but this day is not it and this amp is not the place for me to start! I need to start small and learn by actually doing stuff and finding how the little bits work, and hearing for myself what happens when a part is changed.

Back when you started, all the components where all probably handwired along a turret board of a valve amp, where the reality gap between a schematic and the board was much less than it is now.

The issue is the ghost fuzz. It is something that must happen in a lot of amps. Someone will know what causes this.  They might look at this thread or join the forum at any time and know what causes it.

Let's be positive out there!


phatt

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #47 on: June 25, 2012, 09:49:00 AM »
Hi Paolo,
OK but Amps don't come much easier to draw up a schematic and learn something very useful.

My best guess is You have DC getting onto the signal.
1/ decoupling interstage cap or 2/ a DC filter cap dying.

Where exactly is any ones guess.
now if you had a schematic ,, :-X  arh well I tried. 8|

If your Amp oscillates then that wire link has made that stage unstable,, wise to back it off.
Save the pain mate,, use pedals for gain. xP

Something that might help get your head around these suckers;
Understand that everytime you look at a schematic you are looking at 2 separate but interlinked circuits.

Amplification works by floating the AC Signal (What you hear) on a DC potential.

All the DC parameters are set so as to pass the best possible AC signal. Guitar Amps are often DC offset or deliberately limited to create distortion of the AC signal for obvious reasons.

With most schematics the Positive DC is at the top with ground under the active element. So looking at a simple transistor stage most of the DC is North South whereas the AC Signal usually goes left to right.

AC signal path is not that hard to work out with common sense but the DC stuff is a little more tricky and requires more knowledge.

Needless to say most problems are related to DC as once setup correctly the AC happens by design.
Phil.

Paolo

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #48 on: June 25, 2012, 12:14:12 PM »

The thing is Phil, even if i could trace out the circuit to a schematic, i wouldn't be any the wiser.

I have schematics for a few projects that i can't do.  One of which is to un-mod an Electar Tube 10 amp back to the Fender Champ it was based on and bypass the underdeveloped, unfinished extras that colour the tone.  This only involves adding a few connecting wires to bypass parts of the circuit and changing a couple of values. This amp is much simpler than the Prince amp circuit and i haven't been ale to fully grasp it. But after this, with the things i'm learning, i think i might be able to do it. (I'll be running that schematic past you guys in the future for your opinion on it's competence, before i electrocute myself!). 
I also have a schematic from Carl Martin to make his trem pedal true by-pass but can't do that either (at the moment).

At the end of the road there will always be gain pedals, but NO PEDAL iv'e heard can do the gain tone of this amp.  It's unique and sounds amazing, except for that damn ghos ....

Paolo

Paolo

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Hi

The amp was stressing me out too much so i had to take a break.... then it broke! :'(

Firstly an update:

GAIN - What i settled on for the resistor in the boost channel was 120 ohm, which gives me a great crunchy overdrive at the top of the dial. I could have done with 150 or 170 for a just breaking up tone but with 120 the extra is there and i can back off the guitar volume a bit.  I'll see how it goes in 'the field' anyway.  The normal channel is sounding great also (it always did).

REVERB - I soldered in some new wires with RCA connections for the accutronics tank i borrowed from another amp, which is in the base of the cabinet. The reverb sounds 100 times better than the fool that was in there originally. It doesn't really go very deep but it's just right for my needs.

GHOST FUZZ - Seems  to have lessened somehow. It only occurs on the gain input at  on 7-10 on the dial but is very faint.  Doesn't get any louder when the volume is turned up either. If the reverb is on though wile using the gain input, it's a different story. It's very much there fighting with the main signal.  Luckily, it's not really there using the normal channel.

HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!!

So ... everything was going great until i put the circuit board back in screwed everything back in place, and was getting ready to fix the reverb tank in the base of the cabinet.  I turned it on and something is wrong.  The whole amp sounds like it's powered by 9v battery on it's last legs.  A splattery distorted quieter tone. 

Curiously though, everything seems ok through the headphone socket (does the headphone socket come straight from the pre-amp?).  I have opened up the amp and checked all the wires and connection and can't see anything.  Before i joined the new RCA phono cables to old ones it was working fine, then i put it all back together and bad sound.  The only thing i could think of is that i had dropped some solder or stray wire fragments on the circuit, but i can't see anything.  Maybe it's linked to the ghost fuzz - something that was on it's last legs has now given up?

It's really frustrating because i'd got it running really nice and thought we'd solved all the problems.

Please does anyone know what happened?
« Last Edit: July 22, 2012, 10:39:35 PM by Paolo »

Roly

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #50 on: July 23, 2012, 10:51:08 AM »
Undo what you just did - go back one whole step to the situation where it was disassembled and working; can you revert to that previous situation?  With what result?
If you say theory and practice don't agree you haven't applied enough theory.

Paolo

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #51 on: July 23, 2012, 12:31:06 PM »
Hi Roly

Everything was working great and the chassis was back in and still working great.  Then i took the chassis  back out to solder some RCA connecting wires to the existing reverb wires, then when i put it together again the problem started.

I've disassembled  the connecting wires i'd done but it still sounds bad. That's the only thing i'd changed since it worked.  I've checked for loose wires, bad connections, etc.  Sounds OK like it used to through the headphones though.

Paolo
« Last Edit: July 23, 2012, 03:19:27 PM by Paolo »

Paolo

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #52 on: July 24, 2012, 12:03:31 PM »
On looking at the amp again, i've noticed a small indentation to the resistor which is connected to the treble control, which i assume is from a droplet of solder, but the treble control is still functioning.  In any case i've ordered a replacement 10 ohm resistor.

Another thing which could have some bearing, is that one of the speaker wires had become disconnected at some point during the connection of the extension reverb wiring.  I've heard that switching on an amp with no speaker connected can blow the output transformer?  I hope this is not the case!!

I can't understand how everything sounds fine through the headphone socket tho?

C'mon people - any ideas.  I'm heartbroken here!

Roly

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #53 on: July 24, 2012, 01:13:16 PM »
It is common for resistors to have bumps and paint voids, little bubbles that have burst and set into a crater-like hollow.  Nothing to worry about I suspect - post close-up.

Ten ohms, in the wiper of the treble pot - you sure it's only ten?

That it sounds fine through the 'phones suggests you may have a speaker problem.

Have you tried running the amp into a different speaker cab?

Similarly, try running another (hopefully known good) guitar amp into your speaker (gently) and see how they compare.

And no, a valve amp must never be driven into an open circuit and must always have a suitable load; transistor amps must never be shorted but are fine open circuit.
If you say theory and practice don't agree you haven't applied enough theory.

Paolo

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #54 on: July 24, 2012, 10:12:30 PM »

Here is a close up of the resistor. I think it's definitely a solder melt.  I was also wrong about the value, it's 100 ohm! I've tried a jump wire in it's place and it made no difference, but there was no sound without anything there.

It's in between the volume and the treble controls.  The burnt end in the photo goes to the centre lug of the treble (ground?) The bottom goes way off the right of the photo to another resistor approximately (for cross reference) in the middle of the board and between the volume and boost pots (if looking at previous photos or the 'x-ray' photo you will see it).

Originally i also thought it was the speaker, but I've tried connecting different speakers and it's the same.  Also following your suggestion, i've just tested the speaker with another amp hooked up and the speaker is fine.

I can't say for certain that the speaker wire didn't touch the speaker, the chassis bottom, etc when it was loose. So it might have shorted something.

This probably has nothing to do with it, but my soldering iron stopped working at the same time as my amp broke!

I'll try and describe the sound in more detail:

The normal input has virtually no clean tone anymore. It's sounds more like the boost input was on full but a really bad version. The decay is splattery and has a bit of tremolo/ocillation breaking up the signal.  The boost input sounds like the ghost fuzz has completely taken over and killed the main signal.

Is there anything i can test with my meter?




phatt

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #55 on: July 25, 2012, 01:03:49 AM »
Depending on the output setup,,I'd hasza guess and say you have blown the output devices.
Small low end SS Amps with internal speaker are often hard wired direct to speaker and have *no Short Circuit Protection* as do other SSAmps.

It only takes a micro second short to blow power devices in Amps like that. :'(

Well now that it is likely stuffed you might like to take the slightly faster long cut to fix it and learn something useful in the process. 8|

Paper , pen and start drawing up a schematic. ;)

Otherwise don't mess with stuff.  :trouble
Phil.

J M Fahey

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #56 on: July 25, 2012, 10:42:49 AM »
Quote
The bottom goes way off the right of the photo to another resistor approximately (for cross reference) in the middle of the board and between the volume and boost pots
This kind of "explanation" really explains nothing.
Please draw a proper schematic.

Roly

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Re: Gain control now clean like master volume in Prince Boogie copy amp.
« Reply #57 on: July 25, 2012, 01:57:01 PM »
Well, because of the colour rendering I can't make sense of that resistor value, looks like violet, black, violet which would be a very non-standard and unexpected 70Megs.  Did you measure it while out of circuit?  Whatever, that's a paint void, albeit a bad one, but not likely to be the cause of any of your troubles.

Accidentally flashing the output to ground is why we generally disconnect things at the source end rather than the load end.

Given that you think you may have shorted the output, and given things seem to have changed dramatically for the worse, it's a fair bet you've blown one of the output transistors.

A simple test, with the speaker disconnected, is to measure the half-rail voltage in see if it is indeed close to half rail, or close to the supply or ground.  However, lacking any sort of circuit...we don't know what sort of output stage we are dealing with, and can direct you to the output half-rail.

Compare this;
http://www.OzValveAmps.org/repairs/solidstateamprepair.htm
... with what you've got, and see if you can find any relationships that make sense.

Plan-B; find somebody locally who knows a bit about electronics to give you a hand, maybe sketch the output stage circuit or at least identify what type of arrangement it uses.
If you say theory and practice don't agree you haven't applied enough theory.